Fixed Displacement Pump For Use For Conveying A Hydraulic Fluid In A Closed Hydraulic Circuit, Closed Hydraulic Circuit, Construction Machine And Method For Operating A Fixed Displacement Pump In A Closed Hydraulic Circuit

ABSTRACT

The present invention relates to a fixed displacement pump for use for conveying a hydraulic fluid in a closed hydraulic circuit, a closed hydraulic circuit, a construction machine and a method for operating a fixed displacement pump in a closed hydraulic circuit.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims priority under 35 U.S.C. § 119 of German Patent Application No. 102019007277.4, filed Oct. 18, 2019, the disclosure of which is hereby incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to a fixed displacement pump for use for conveying a hydraulic fluid in a closed hydraulic circuit, a closed hydraulic circuit, a construction machine and a method for operating a fixed displacement pump in a closed hydraulic circuit.

BACKGROUND OF THE INVENTION

Fixed displacement pumps are known and already well-established in the field of hydraulics. In contrast to variable displacement pumps, fixed displacement pumps are characterized by the fact that they displace a constant volume with each revolution. Such a fixed displacement pump is disclosed, for example, in DE2307351A. Typical elements of a generic fixed displacement pump include a pump housing, at least one conveying element arranged inside the pump housing in a conveying chamber and rotatably driven about a rotation axis for conveying purposes, the conveying element having at least one bearing axle extending in axial direction of the rotation axis, the conveying element being supported for rotation with the bearing axle about the rotation axis within an axle bearing provided in axial direction next to the conveying element and configured stationary relative to the pump housing, a fluid pump inlet via which hydraulic fluid to be conveyed can be conveyed from outside the fixed displacement pump into the conveying chamber on a low-pressure side, and a fluid pump outlet via which conveyed hydraulic fluid from the conveying chamber can be discharged from the fixed displacement pump on a high-pressure side. The pump housing refers to the unit that shields the pump from the outside environment. The pump housing may also have bearing devices inside, in particular for forming an axle bearing. The conveying element is, in particular, a gearwheel, more particularly a pair of meshing gearwheels. The conveying element is rotatable about the rotation axis inside the conveying chamber, the rotational movement of the conveying element within the conveying chamber causing the hydraulic fluid to be conveyed from the fluid pump inlet side to the fluid pump outlet side within the fixed displacement pump. If this pump is integrated in a hydraulic system, a fluid pressure is built up by the connected consumer(s) on the fluid pump outlet side, so that this side (high-pressure side), on which the conveyed fluid leaves the fixed displacement pump, has an increased pressure level compared to the fluid at the fluid pump inlet (low-pressure side).

During operation of such a fixed displacement pump, leakage flows frequently occur. A portion of the hydraulic fluid then escapes from the conveyed flow of the hydraulic fluid, for example between the axle bearing and the bearing axle of the conveying element, and is therefore not conveyed to the high-pressure side by the conveying element. For fixed displacement pumps, it is known in this context to provide a device for draining off a leakage portion of the hydraulic fluid escaping from the conveying chamber via the axle bearing. Such a device may consist, for example, in a groove leading from the axle bearing to the low-pressure side of the fixed displacement pump opposite the conveying chamber. If such a fixed displacement pump is operated in an open hydraulic circuit, the pressure on the low-pressure side is considerably lower than on the high-pressure side (usually up to a maximum of 3 bar) or even negative, so that the low-pressure side, where hydraulic fluid enters the fixed displacement pump, is in this case also referred to as suction side. The leakage oil itself is then sucked in by the fixed displacement pump on the suction side and returned to the conveying chamber. The limiting component at this point is the shaft sealing ring of the drive shaft.

However, such an arrangement is not suitable for use in a closed hydraulic circuit, because then, for example, the suction effect described above does no longer occur, i.e., the very low pressure level on the suction side no longer exists. In a closed hydraulic circuit, the hydraulic fluid has a relatively high pressure even on the low-pressure side, for example at least above 10 bar, usually in the range of 20 to 30 bar, so that the leakage oil can no longer be sucked off and/or the hydraulic fluid is instead pressed into the axle bearing via the grooves or similar devices and/or passes through/damages the shaft sealing ring, so that oil escapes from the fixed displacement pump. A closed hydraulic circuit is thus characterized by the fact that the hydraulic fluid from the hydraulic consumer is not drained off into a tank but is returned to the pump in one or more lines. The pump is thus “clamped” into the hydraulic circuit on both sides. A defined discharge of leakage oil towards a suction or low-pressure side as described above is then no longer possible.

For this reason, axial piston pumps are normally used in closed hydraulic circuits. Such hydraulic pumps allow for a variable conveying volume per revolution of the respective conveying element of the pump. However, such pumps are relatively cost and maintenance intensive.

Against this background, one aspect of the present invention is to improve a fixed displacement pump so as to make it suitable for use in a closed hydraulic circuit. Ideally, it should also be possible to vary the conveying volume and/or even reverse the conveying direction in the closed hydraulic circuit.

SUMMARY OF THE INVENTION

One aspect of the present invention consists in the fact that the fixed displacement pump has a collection chamber adjacent to the axle bearing of the fixed displacement pump for the leakage portion of the hydraulic fluid escaping from the axle bearing, wherein a sealing device is provided which seals the collection chamber against the low-pressure side and the high-pressure side of the fixed displacement pump, and that the fixed displacement pump has a leakage oil outlet connected to the collection chamber for discharging the leakage oil, in particular separately, from the fixed displacement pump. The collection chamber thus no longer has a direct line connection to the low-pressure side of the fixed displacement pump, but a separate leakage oil outlet. Thus, in the fixed displacement pump according to the present invention, the leakage oil is no longer fed to the hydraulic circuit on a suction side directly within the pump but is drained off from the fixed displacement pump via a separate leakage oil outlet which connects the collection chamber to a leakage oil outlet opening of the fixed displacement pump. Moreover, the sealing device seals not only the high-pressure side of the fixed displacement pump against the collection chamber but, according to the present invention, additionally also the low-pressure side, so that the hydraulic fluid is also prevented from urging into the collection chamber from the low-pressure side. The task of the collection chamber is to collect leakage portions escaping from the axle bearing in a controlled manner. In this regard, it is preferred if the collection chamber is configured such that it combines the leakage oil escaping from several axle bearings and thus enables the leakage oil to be discharged centrally from the fixed displacement pump.

It has now turned out to be advantageous if the collection chamber is configured as a collection chamber that is pressureless with respect to the outside environment during operation of the fixed displacement pump. This enables a pressureless discharge of the leakage oil accumulating in the collection chamber from the pump. There is therefore no increased or decreased pressure level in the collection chamber compared to the outside environment of the fixed displacement pump, but the pressure of the outside environment.

With regard to the specific design of the collection chamber, a variety of advantageous variations are possible, which may also be combined. It is, for example, preferred if the collection chamber extends through components that are stationary relative to the pump housing and/or stationary relative to the at least one bearing axle. Stationary in this context means that the walls delimiting the collection chamber at least partially do not change their position relative to the pump housing or the bearing axle during operation of the fixed displacement pump. A collection chamber that is stationary relative to the pump housing, may, in particular, also be formed directly by a pump housing component itself. A component that is stationary relative to the bearing axle, may, in particular, also be formed directly by the bearing axle itself. It is such essential for such a collection chamber that it rotates about the rotation axis together with the bearing axle. However, the collection chamber, may, in particular, also be configured such that it has areas that are stationary with respect to both the pump housing and the bearing axle. In one exemplary design, the collection chamber has an annular space which surrounds the bearing axle, in particular completely. Additionally or alternatively, it may also comprise a collection head section directly adjoining a face side of the bearing axle in axial direction of the bearing axle, in particular such that the bearing axle and the axle bearing terminate flush with each other at the face side towards the collection chamber, or the bearing axle protrudes in axial direction slightly beyond the axle bearing into the collection chamber. Additionally or alternatively, the collection chamber may have a collection passage opening passing completely through the bearing axle in axial direction from one face side to the other. This arrangement, may, in particular, be used to connect two areas of the collection chamber that are stationary relative to the pump housing. Further, the collection chamber may additionally or alternatively also be configured such that it has a connection channel that connects an annular space surrounding a bearing axle, or a collection head section, or a collection passage opening, with an annular space surrounding a bearing axle, or a collection head section, or a collection passage opening. Such a connection channel is, in particular, configured stationary relative to the pump housing, for example as a connection bore. It is further advantageous if the collection chamber comprises a manifold into which multiple connection channels lead. Multiple leakage oil flows are thus combined centrally in the manifold via the connection channels.

It is now possible to form the collection chamber partially in multiple components and/or across multiple components and/or in multiple areas of the fixed displacement pump. In this case, the collection chamber thus extends in and/or across multiple components. For this, it is particularly suitable in one embodiment, for example, to form the collection chamber, or at least a subregion thereof, in the bearing axle, in particular such that it passes through the bearing axle in axial direction, in particular completely. Additionally or alternatively, the collection chamber may also be formed in the conveying means, in particular directly, and, in particular, also such that it passes through the bearing axle in axial direction of the rotation axis, in particular completely. Another option is to form the collection chamber at least partially in a bearing rest in which at least one bearing axle is supported. Such a bearing rest, especially if formed as one piece, may, in particular, be intended to simultaneously support two parallel bearing axles of a fixed displacement pump, for example as a connecting part between two pump units of a fixed displacement pump configured as a tandem pump. However, it is generally also possible to form such a bearing rest in multiple pieces. Additionally or alternatively, the collection chamber may further also be formed, for example, in a pump flange or a pump flange plate, in particular as a surface recess on the side opposite a flanging side. Additionally or alternatively, the collection chamber may also be formed in a pump cover, in particular as a surface recess on the side opposite an outside of the pump cover. Additionally or alternatively, another component for forming the collection chamber is a coupling plate of a tandem pump. Such a coupling plate serves to mechanically couple two fixed displacement pump units in a tandem arrangement known per se in the prior art. In terms of construction, such a coupling plate may be configured, for example, as a plate with two or more opposing recesses and/or passage openings for receiving at least one bearing axle of each fixed displacement pump unit, enabling a comparatively compact tandem arrangement, or at least with a through-opening for a coupling element for drive purposes between the two fixed displacement pump units.

There are also variation possibilities with regard to the specific design of the sealing device. The latter may be arranged, for example, as a contact seal, in particular between a pump cover and a housing part which, in particular, forms the radial conveying chamber relative to the rotation axis of the at least one conveying element. The sealing device may consist of an elastic material. The material of the sealing device, may, in particular, be a polymer material, in particular a plastic polymer, for example NBR (nitrile rubber, fluorine rubber, etc.). Elastomers or thermoplastics are preferred in one embodiment. The use of metallic seals is also possible. It is also possible to use coated seals as sealing device. It is particularly possible to configure the sealing device as a flat gasket, i.e., in a design in which the height of the seal is small compared to the sealing surface and which, in particular, adapts to the adjacent sealing surfaces with its entire width of its sealing surface. As an alternative, the sealing device may also be configured, for example, as a molded seal, especially with a rather square cross-section with rounded edges. The sealing device may consist of multiple separate subunits. However, it may be advantageous, for example to facilitate assembly, if the sealing device as a whole is formed in one piece. The sealing device used to simultaneously seal the collection chamber from the low-pressure side and the high-pressure side of the fixed displacement pump then consists of a single, continuous part. It may further also be possible that the sealing device is composed of multiple subunits, but then preferably of subunits of identical construction according to one embodiment. Additionally or alternatively, it may have at least one sealing section which completely surrounds the bearing axle or the rotation axis of the conveying element radially. Particularly in the case where the fixed displacement pump comprises more than one conveying element, the sealing device may be configured such that it comprises two sealing sections, each of which completely surrounds a bearing axle radially, as well as a connecting section which connects the two sealing sections. For this, the sealing device, may, in particular, be configured as a sealing rest. The sealing device, may, in particular, comprise at least two sealing elements which are arranged one behind another, in particular congruently to one another, in axial direction of the rotation axis of a conveying element. Further, the sealing device may be subjected to a clamping force in axial direction of the rotation axis.

Modifications are also possible with regard to the specific design of the fixed displacement pump. The fixed displacement pump is preferably an external gear pump comprising two adjacent, meshing gearwheels with a respective external toothing which rotate about a respective rotation axis, and two parallel bearing axles arranged at least partially at a same height in axial direction of the rotation axes, one bearing axle carrying one gearwheel and the other bearing axle carrying the other gearwheel. It is advantageous if one of the bearing axles is designed as a drive axle protruding from the housing, comprising a positive locking device for engagement of a drive arrangement, in particular a spline gearing. This enables the connection of a drive train. Additionally or alternatively, only one of the bearing axles, in particular the bearing axle driven by the other bearing axle, comprises a passage opening extending in axial direction completely through the bearing axle and open at both face sides as part of the collection chamber. In contrast, the bearing axle connected to the drive train external to the fixed displacement pump is preferably of solid construction.

It is known and common for construction machines in particular that they may have multiple functionally separate driven hydraulic components at the same time, such as, in particular, for road pavers and feeders. It is therefore possible according to one embodiment if the fixed displacement pump as a tandem pump is configured with a first and a second fixed displacement pump unit. In this manner, multiple separate hydraulic circuits can be operated simultaneously via a drive train driving the fixed displacement pump with the multiple fixed displacement pump units. According to one embodiment of the present invention, the tandem pump may have a coupling plate for this purpose which is arranged directly between two gearwheel pairs of the first and second fixed displacement pump units. It is now provided that a part of the collection chamber of the fixed displacement pump is formed by the coupling plate and/or that a respective seal is provided as part of the sealing device on both sides of the coupling plate in the direction of the rotation axis of the gearwheel pairs. Additionally or alternatively, multiple bearing axles may have passage openings passing completely through them as part of the collection chamber. This makes it easier to achieve a common, continuous collection chamber for all the fixed displacement pump units of the fixed displacement pump. Additionally or alternatively, such a fixed displacement pump with multiple fixed displacement pump units comprises, in particular exclusively, a single common leakage oil outlet. Through this, the leakage oil of the at least two fixed displacement pump units is thus discharged into one contiguous collection chamber and drained off from the fixed displacement pump via said single common leakage oil outlet. In this regard, it is possible for the leakage oil outlet to be arranged in a pump cover, in particular a pump cover of only one of the at least two fixed displacement pump units. Additionally or alternatively, the leakage oil outlet may be arranged in a flange.

According to another embodiment of the present invention, the fixed displacement pump may comprise a valve device connected to the fluid pump inlet and the fluid pump outlet and/or arranged upstream of the fluid pump inlet and the fluid pump outlet. With the aid of such a valve device, functionalities can be imitated for the fixed displacement pump, which is operated, in particular, in a closed circuit, which are conventionally only possible, for example, with a variable displacement pump with variable conveying volume, such as, for example, a reversal of the flow direction in at least a part of the closed hydraulic circuit and/or an adjustment of the hydraulic fluid volume per revolution of the fixed displacement pump conveyed in at least a part of the closed hydraulic circuit. A valve function of the valve device may comprise “switching between direct and closed bypass from the fluid pump inlet to the fluid pump outlet and an interruption of the closed bypass”. If the hydraulic fluid is routed directly from the fluid pump outlet to the fluid pump inlet, the hydraulic fluid conveyed by the fixed displacement pump does not pass through any hydraulic consumer in the closed circuit. In this valve function, the fixed displacement pump thus conveys hydraulic fluid in a shortened closed hydraulic circuit bypassing at least one hydraulic consumer, such as a hydraulic motor, from the fluid pump outlet via the valve device directly back to the fluid pump inlet. Accordingly, the hydraulic consumer is then inactive. Thus, if the closed direct bypass via the valve device from the fluid pump outlet to the fluid pump inlet is activated, no hydraulic fluid is conveyed to the hydraulic consumer in the closed hydraulic circuit. Additionally or alternatively, the valve device may also be configured such that it enables switching a fluid connection between the fluid pump outlet and a first outlet of the valve device to a fluid connection between the fluid pump outlet and a second outlet of the valve device. Through this switching constellation, it is possible, for example, to achieve a reversal of the conveying direction in that part of the closed hydraulic circuit that is between the valve device and the hydraulic consumer, for example a hydraulic motor. Additionally or alternatively, the valve device may also be configured such that it enables the hydraulic volume conveyed by the fixed displacement pump to be divided into two partial flows, more specifically a partial flow returned directly to the fixed displacement pump and a partial flow forwarded to the hydraulic consumer. In this manner, the portion of the hydraulic fluid volume forwarded to the hydraulic consumer can be varied in the sense of a variable displacement pump even with a constant conveying volume of the fixed displacement pump. Moreover, the valve device may also be configured such that a pressure protection of a high-pressure side of the fixed displacement pump and/or release of a discharge line is effected when a pressure threshold on the high-pressure side is exceeded. For this, the valve device may comprise, for example, a pressure-limiting valve, which is preferably adjustable. Additionally or alternatively, it is further possible that the valve device is configured such that it enables establishing a pressure-protected fluid connection from a feed line to a low-pressure area of the fixed displacement pump. Such an arrangement services, in particular, to compensate for leakage oil losses and/or heat-related oil losses in the closed hydraulic circuit in order to enable a desired fluid volume in the closed hydraulic circuit and thus a desired pressure level, in particular also on the low-pressure side of the closed hydraulic circuit. For this, the feed line may be supplied with hydraulic fluid, for example, by a suitable feed pump. The feed line is connected to the closed hydraulic circuit, towards the low-pressure side or the high-pressure side, via a suitable pressure protection device, for example a pressure-limiting valve, which is, in particular, adjustable. The feed pump may constitute a unit which is separate from the fixed displacement pump according to the present invention. In another alternative, however, the feed pump may also be integrated as an additional stage in the fixed displacement pump according to one embodiment of the present invention.

In terms of the structural conception, there are also various possibilities for an advantageous configuration of the valve device. In particular, the valve device may be configured as a modular unit with a basic valve block and one or more functional valve blocks. The basic valve block may, for example, comprise a connection opening for connection to the fluid pump inlet or a line to the fluid pump inlet of the fixed displacement pump, and/or a connection opening for connection to the fluid pump outlet or a line to the fluid pump outlet of the fixed displacement pump, and/or a connection opening for connection to a motor inlet or a line to the inlet of a motor, in particular a hydraulic motor, and/or a connection opening for connection to a motor outlet or a line to the outlet of a motor, in particular a hydraulic motor. The basic valve block may additionally comprise a connection opening for connection to a feed line. Via this connection, it is thus possible to feed hydraulic fluid into the closed hydraulic circuit from an external source, for example with the aid of a feed pump. Additionally or alternatively, the basic valve block may have a line interruption device with an interruption outlet opening and an interruption inlet opening. The interruption outlet opening and the interruption inlet opening are not in fluid connection in the basic valve block itself but may be brought into fluid connection in the manner described above with the aid of a suitable functional module block.

To achieve various functionalities, one or more functional valve blocks are preferably provided in addition to the basic valve block. With a functional valve block, one or more operational functions may be obtained at the basic valve block. The configuration as a separate and functional module block interchangeably arranged on the basic valve block allows individual adjustment of the specific functions of the valve device. For this purpose, the functional module block has a module block inlet opening and a module block outlet opening which is in fluid communication with the module block inlet opening via a valve device and/or a line connection. The hydraulic fluid thus passes through the functional module block from the inlet opening to the outlet opening. The functional module block may, for example, be attached, in particular interchangeably, to the basic valve block such that the interruption outlet opening is in fluid connection with the module block inlet opening and the module block outlet opening is in fluid connection with the interruption inlet opening. With this arrangement, the hydraulic fluid, in particular coming from the fixed displacement pump in the closed hydraulic circuit, is routed from the basic valve block into the functional module block and thence back to the basic valve block before being supplied to a hydraulic consumer, for example a hydraulic motor. The basic valve block thus constitutes a type of adapter supplied by the fixed displacement pump with hydraulic fluid in a closed system, with which various functions can be implemented. For this, at least one functional module block is connected to the basic valve block. The functional module block may accordingly have pure passage lines for the hydraulic fluid and/or, in particular, switchable switching devices, in particular at least one valve, more particularly at least one 2/2-way valve and/or at least one 4/2-way valve, and/or an, in particular continuously variable, flow divider, specifically a variable, in particular proportionally controllable, flow control valve.

Specifically, the functional module block may, for example, be configured such that it enables switching between two inlet and two outlet openings, in particular, for example, via a switchable 4/2-way valve. With this function a closed hydraulic circuit routed through the functional module block may be reversed with respect to its flow direction towards a consumer. Additionally or alternatively, the functional module block may also be configured as a pass-through bridge, thus enabling static pass-through from defined inlets to defined outlets. Additionally or alternatively, the functional module block may further also be configured such that it can be used to switch on and off a feeding function through which hydraulic fluid is fed into the closed hydraulic circuit from outside. Additionally or alternatively, the functional module block may further divide an incoming hydraulic volume flow into two outgoing hydraulic volume flows, for example with the aid of a flow control valve. In this manner, it is, for example, possible to vary the volume flow portion forwarded in the flow direction to a hydraulic consumer and the volume flow portion returned to the pump, for example by bypassing. Another modification presented using a functional module block may also be a feed pressure bypass, which is particularly useful for the cold start phase. With the aid of the feed pressure bypass, hydraulic fluid may be rerouted directly to the fixed displacement pump, bypassing the consumer. The hydraulic fluid in the closed hydraulic circuit is thus circulated only in a subsegment and can thus heat up more quickly. Only when the feed pressure bypass is blocked will the fixed displacement pump convey into the line section located in the conveying direction downstream of the fixed displacement pump towards the consumer. Such a functionality may be provided with a suitable bypass line, for example also in the basic module block, and a suitable valve, in particular in the functional module block. It is essential for the bypass line that it enables the hydraulic fluid conveyed by the fixed displacement pump to be returned from the high-pressure side to the low-pressure side bypassing the consumer.

There are also variation possibilities with regard to the specific configuration of the basic valve block. For example, the basic valve block may be configured for simultaneous connection of at least two functional module blocks. The connected functional module blocks may implement different functionalities. Generally, the two functional module blocks may be configured such that they are configured for performing at least one of the functionalities “on/off function” and/or “reversal of direction” and/or “flow divider” or “flow control” at a connected consumer, in particular a hydraulic motor, wherein the respective functionality refers to the behavior of the connected consumer driven by the closed hydraulic circuit.

Irrespective of the above statements, the valve device, in particular the basic valve block, may be configured such that it has a connection for a feed pump, and that it has an, in particular pressure-protected, switching device for discharging hydraulic fluid from the closed circuit into a discharge line leading to a line connection, for example due to thermal effects and/or for partial cooling of the hydraulic fluid circulating in the closed hydraulic circuit. In this way, part of the hydraulic fluid can be extracted from and supplied to the closed hydraulic circuit in a controlled manner from outside the closed hydraulic circuit. This may serve, in particular, to compensate for leakage oil losses and/or for cooling purposes of the hydraulic fluid volume circulating in the closed hydraulic circuit. Such a pressure-protected switching device is, for example, a pressure-limiting valve.

Generally, the valve device may be configured as a unit that is structurally connected to the fixed displacement pump or as a separate unit that is in fluid connection with the fixed displacement pump. The present invention also relates to these two variants.

A further aspect of the present invention relates to a closed hydraulic circuit comprising a fixed displacement pump according to the present invention, in particular with a valve device, as described above. In addition to a fixed displacement pump according to the present invention, the closed hydraulic circuit thus has a hydraulic consumer supplied with hydraulic fluid by the fixed displacement pump, in particular a hydraulic motor, with a fluid motor inlet and a fluid motor outlet, a hydraulic fluid supply line connecting the fluid pump outlet to the fluid motor inlet, and a hydraulic fluid return line connecting the fluid motor outlet to the fluid pump inlet.

The present invention also relates to a construction machine, in particular a road paver, a feeder, a road milling machine, a stabilizer, a recycler, a landfill or ground compactor, a ground or road roller, with at least one closed hydraulic circuit according to the present invention.

A further aspect of the present invention finally relates to a method for operating a fixed displacement pump operated in a closed hydraulic circuit, in particular a fixed displacement pump according to the present invention. The method according to one embodiment of the present invention initially comprises conveying hydraulic fluid with the aid of the fixed displacement pump towards a high-pressure side of the closed hydraulic circuit. By means of this conveying, an increased pressure level (“high-pressure side”) compared to the low-pressure side is generated in the closed hydraulic circuit between the fluid outlet at the fixed displacement pump and the at least one hydraulic consumer, which is maintained during operation. As a result, in a further step, a hydraulic consumer, in particular a hydraulic motor, is driven by the hydraulic fluid on the high-pressure side. Further, the hydraulic fluid is returned from the hydraulic consumer, in particular the hydraulic motor, to the fixed displacement pump on the low-pressure side of the closed hydraulic circuit. Except for the use of a fixed displacement pump, this corresponds to the typical procedure in a closed hydraulic circuit. In addition to the above steps, according to an exemplary embodiment of the present invention, leakage oil is now simultaneously collected within the fixed displacement pump in a collection chamber separate from the low-pressure side and the high-pressure side of the closed hydraulic circuit. Reference is made to the above statements with respect to the possible structure of this collection chamber. What is essential is that a collection chamber which is sealed against both the high-pressure side and the low-pressure side is used for the collection, and that the leakage oil collected via the collection chamber is not fed to a suction side of the fixed displacement pump. Since the method according to one embodiment of the present invention provides for operation of the fixed displacement pump in a closed hydraulic circuit, there is no suction side on the fixed displacement pump. According to one embodiment of the present invention, it is therefore further provided that the leakage oil is drained off from the fixed displacement pump, in particular separately from the hydraulic fluid circulating within the closed hydraulic circuit, which results in a volume portion of hydraulic fluid being removed from the closed hydraulic circuit. The draining off may be carried out in a pressureless manner, i.e., without a pressure difference compared to the outside environment of the fixed displacement pump. In addition, the leakage oil may be drained off from the fixed displacement pump via a leakage oil outlet of the fixed displacement pump, in particular a single one. The method according to one embodiment of the present invention finally further comprises feeding hydraulic fluid into the closed hydraulic circuit to compensate for the leakage oil branched off from the closed hydraulic circuit in the preceding steps in order to keep the total volume of hydraulic fluid in the closed hydraulic circuit at least essentially as constant as possible. For this, a suitable feeding device may be provided, in particular with a feed pump separate from the fixed displacement pump according to the present invention, which may likewise be a fixed displacement pump.

For performing the method according to one embodiment of the present invention, it is preferred if leakage oil is collected at least partially via a passage opening arranged in a bearing axle. The collecting is thus performed at least partially via a subregion of the collection chamber rotating about the rotation axis with the conveying element. Ideally, the leakage oil is collected via a collection chamber which rotates at least partially about the rotation axis with the conveying element and, in particular, extends through the latter, more particularly along the rotation axis, and which has a subregion which is stationary relative to a pump housing of the fixed displacement pump, in particular in the region of the leakage oil outlet.

In order to be able to collect leakage oil escaping via the axle bearing particularly effectively, for collecting leakage oil in the collection chamber the collection chamber may be sealed against both the high-pressure side and the low-pressure side of the fixed displacement pump with the aid of a sealing device, in particular with the aid of a sealing device as described in more detail for the fixed displacement pump according to one embodiment of the present invention.

It may be advantageous if the fixed displacement pump as a tandem pump is configured with a first fixed displacement pump unit and a second fixed displacement pump unit. With this particular arrangement, leakage oil quantities may be collected separately for each fixed displacement pump unit. However, it is possible to combine the leakage oil flows of the first and second fixed displacement pump units and then to drain off the combined leakage oil flows from the tandem pump together to avoid having to drain off two separate leakage oil flows. Additionally or alternatively, the two closed hydraulic circuits operated via the two fixed displacement pump units are supplied with hydraulic fluid via a common feed pump which has supply lines to both closed hydraulic circuits.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be described in more detail below by reference to the embodiment examples shown in the figures. In the schematic figures.

FIG. 1 is a side view of an exemplary construction machine, in this case a road paver;

FIG. 2 is a schematic diagram showing a closed hydraulic circuit;

FIG. 3 is a schematic diagram showing the mode of operation of a fixed displacement pump of the external gear pump type;

FIG. 4 is an oblique perspective view of a fixed displacement pump with two fixed displacement pump units;

FIG. 5a is a cross-sectional view through the fixed displacement pump units of FIG. 4 in the plane I in FIG. 4 (viewing direction along arrow I) in a modified fixed displacement pump;

FIG. 5b shows the cross-sectional view of FIG. 5a with highlighted collection chamber;

FIG. 6 is a cross-sectional view through a fixed displacement pump in a perspective comparable to plane II in FIG. 4 (viewing direction according to arrow II) onto a prior art sealing device;

FIG. 7 is a cross-sectional view through the fixed displacement pump of FIG. 4 in the plane II in FIG. 4 (viewing direction according to arrow II) onto a sealing device according to the present invention;

FIG. 8 is a hydraulic circuit diagram of a closed hydraulic circuit;

FIG. 9a is a hydraulic circuit diagram of a module of a valve device with on/off and direction reversal function;

FIG. 9b is a hydraulic circuit diagram of a module of a valve device with on/off function;

FIG. 9c is a hydraulic circuit diagram of a module of a valve device with stepless on/off and direction reversal function;

FIG. 9d is a hydraulic circuit diagram of a module of a valve device with stepless on/off function;

FIG. 10 shows another variation of the hydraulic circuit diagram of FIG. 9 a;

FIG. 11 is a flow chart of a method according to the present invention.

Like components are designated by like reference numerals in the figures, although recurring components may not be designated separately throughout the figures.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows an example of a construction machine 1 which is particularly suitable for use of the arrangement according to the present invention, or use of the method according to the present invention, as respectively described in more detail below. The construction machine 1 according to FIG. 1 is specifically a so-called road paver, elements of which are a machine frame 2, traveling devices 3, an operator platform 4, a drive motor 5, a material hopper 6, as well as a paving screed 7. The structure and the mode of operation of such a construction machine 1, in particular the road paver shown in FIG. 1, are known per se. During working operation, the construction machine moves over the underlying ground in or against the traveling direction A (in the case of a road paver, the direction A is at the same time its paving direction). Modern construction machines, in particular road pavers, usually comprise a plurality of hydraulic consumers, such as longitudinal and transversal conveyor devices (not shown in FIG. 1), compaction devices, in particular at the paving screed or in a roller drum, etc. Another hydraulic consumer, which is, in particular, also typical for self-propelled construction machines, is a travel drive in the form of a hydraulic motor. To drive these hydraulic consumers, at least one hydraulic pump (not shown in FIG. 1) is provided which conveys hydraulic fluid to the respective hydraulic consumer(s) in a manner known per se. The present invention according to one embodiment is directed to arranging the hydraulic pump and the at least one hydraulic consumer in a closed hydraulic circuit and resorting to a fixed displacement pump in the manner described in more detail below.

FIG. 2 illustrates the basic structure of a closed hydraulic circuit 8 according to an exemplary embodiment of the present invention. Elements of such a closed hydraulic circuit 8 include a pump configured as a fixed displacement pump 9 with a pump outlet 10 and a pump inlet 11, as well as a hydraulic consumer 12, in particular a hydraulic motor, with a motor inlet 13 via which hydraulic fluid enters and drives the hydraulic consumer, and with a motor outlet 14 via which the hydraulic fluid leaves the hydraulic consumer 14. Via a supply line 15, hydraulic fluid conveyed by the fixed displacement pump 9 is conveyed from the pump outlet 10 to the motor inlet 13. This subregion of the hydraulic circuit 8 is also referred to as the high-pressure side. Hydraulic fluid exiting the hydraulic consumer 12 in the closed hydraulic circuit 8 is supplied back to the fixed displacement pump 9 via a return line 16 which connects the motor outlet 14 to the pump inlet 11. The subregion of the closed hydraulic circuit 8 between the motor outlet 14 and the pump inlet 11 is also referred to as the low-pressure side. Depending on the configuration of the fixed displacement pump 9, it is possible that the latter is switchable with regard to its conveying direction. Additionally or alternatively, it is also possible to use a valve device 19 to effect a reversal of the flow direction only in the section of the closed hydraulic circuit 8 between the valve device 19 and the hydraulic consumer 12. It will be appreciated that in such a case the designations pump outlet 10, pump inlet 11, motor inlet 13, motor outlet 14, supply line 15 and return line 16 will switch accordingly. The fixed displacement pump 9 may be connected to a drive element, for example a combustion engine or a pump transfer gear, via an input shaft 17. The hydraulic consumer 12 may drive an output shaft 18 via which an element to be driven, such as a conveyor device, a travel drive, an exciter, etc., is set in motion.

In the present embodiment example, the closed hydraulic circuit 8 may further comprise the valve device 19. The latter may have a basic valve block 20 and two functional module blocks 21 attached to it, for example. The hydraulic fluid conveyed by the fixed displacement pump 9 via the pump outlet 10 first passes through the basic valve block 20 and then the hydraulic consumer 12. Via the motor outlet 14, the conveyed hydraulic fluid is then returned to the fixed displacement pump 9, again passing through the basic valve block 20 in the closed hydraulic circuit 8.

Part of the fixed displacement pump 9 is a collection chamber 22 in which leakage oil occurring during operation of the fixed displacement pump 9 is collected. The leakage oil collected inside the fixed displacement pump 9 may be discharged from the interior of the fixed displacement pump 9 via a leakage oil outlet 23, in particular in a pressureless manner with respect to the outside environment of the fixed displacement pump 9. For this, a discharge line 24 connected to the leakage oil outlet 23 may be provided which drains off the collected leakage oil from the leakage oil outlet 23 into a tank 25.

Finally, a feeding device 26 may be provided to compensate for the leakage oil losses in the closed hydraulic circuit 8. This feeding device 26 may comprise a feed pump 27 separate from the fixed displacement pump 9 of the closed hydraulic circuit 8, for example a conventional fixed displacement pump in an open hydraulic circuit, which is connected via a feed line 28 to the closed hydraulic circuit 8 and/or one of its components, such as in the present embodiment example the valve device 19, such that hydraulic fluid can be fed into the closed hydraulic circuit 8 to compensate for the leakage oil losses. Further details of the possible basic concept of an exemplary closed hydraulic circuit 8 according to the present invention as outlined in FIG. 2 will be described in the following figures.

FIG. 3 shows a cross-section through an exemplary fixed displacement pump 9 for a more detailed illustration of a possible conveying principle of the fixed displacement pump, as it may be used, in particular, to implement a fixed displacement pump according to an exemplary embodiment of the present invention. The flow direction of the hydraulic fluid conveyed by the fixed displacement pump 9 is indicated with arrows in FIG. 3. In the embodiment example shown in FIG. 3, the fixed displacement pump 9 is an external gear pump, although the present invention generally also relates to other specific configurations of the fixed displacement pump 9 with different conveying principles. Via the pump inlet 11, hydraulic fluid enters the interior of a pump housing 29 of the fixed displacement pump 9. The conveying rate of the hydraulic fluid is in this case generated, for example, by two conveying elements 30 configured as adjacent, meshing external gearwheels. These conveying elements rotate about the two parallel rotation axes R inside a conveying chamber 38 which is rounded at the opposite sides of the two external gearwheels and extends longitudinally along the two gearwheels. The pump inlet 11 (“low-pressure side”) and the pump outlet 10 (“high-pressure side”) are located opposite each other in the longitudinal side areas. As a result of the rotation of the two gearwheels, the hydraulic fluid is conveyed from the pump inlet 11 to the pump outlet 10 in the opposite outer area. The conveying elements 30, which can rotate about the respective rotation axis R, are in each case supported for rotation on a bearing axle 31 on the pump housing 29 via an axle bearing (not shown in FIG. 3). During operation of the fixed displacement pump 9, as shown, for example, in FIG. 3, leakage oil flows may now occur as a result of this support of the conveying elements 30, so that a portion of the hydraulic fluid escapes from the closed hydraulic circuit 8. In the present application of a fixed displacement pump 9 in a closed hydraulic circuit 8, this is challenging in that there is no suction side or negative pressure at the pump inlet 11 due to the use of a closed hydraulic system, but rather a low-pressure level (“low-pressure side”) typical for closed hydraulic circuits 8, for example in the range of 20 to 40 bar (compared to clearly >100 bar, in particular >200 bar, on the high-pressure side). A recirculation of leakage oil flows occurring during operation of the fixed displacement pump 9 via a feed on a suction side as in conventional fixed displacement pumps is therefore not possible. The fixed displacement pump 9 therefore has the collection chamber 22 described in more detail below for collecting and combining leakage oil flows separately and sealed against the low-pressure side and the high-pressure side.

FIG. 4 shows a fixed displacement pump 9 in a tandem arrangement with two fixed displacement pump units 9A and 9B. On the side of the input shaft 17, a mounting flange 37 is provided via which the fixed displacement pump 9 can be flanged to a drive element, for example a drive motor or a pump transfer gear. The two fixed displacement pump units 9A and 9B are arranged in series, so that only one input shaft 17 is required for driving the two fixed displacement pump units 9A and 9B. Each of the two fixed displacement pump units 9A and 9B supplies a respective closed hydraulic circuit (not shown in FIG. 4). The two pump inlets 11 are arranged on the front side of the fixed displacement pump 9 in FIG. 4. On the rear side facing away from the viewer, there are two pump outlets (not visible in FIG. 4). On the side of the pump housing 29 opposite the input shaft 17, the two fixed displacement pump units 9A and 9B, or the fixed displacement pump 9, have a common leakage oil outlet 23. All the leakage oil of both fixed displacement pump units 9A and 9B collected in the collection chamber described in more detail below is discharged from the fixed displacement pump 9 via this exclusively one leakage oil outlet 23 of the fixed displacement pump 9. However, a separate collection and discharge for each of the two fixed displacement pump units 9A and 9B is also possible. The pump housing 29 is formed in multiple pieces and comprises two housing sleeves 29A and 29B, which are each assigned to a fixed displacement pump unit 9A and 9B, a flange plate 29C with the mounting flange 37, and a coupling plate 29A, via which the housing sleeves 29A and 29B of the two fixed displacement pump units 9A and 9B are connected to one another. Via the face side opposite the leakage oil outlet 23 and the flange plate 29C, connecting screws 32 are further provided which pass through the two housing sleeves 29A and 29B and the coupling plate 29D up to the flange plate 29C and are in threaded engagement in the flange plate 29C. With the aid of the connecting screws 32, a contact pressure acting in axial direction of the rotation axes of the conveying elements, in particular between the elements 29A, 29B, 29C and 29D, can thus be generated which can be used for sealing purposes in the manner described below. It is generally also possible that the leakage oil outlet 23 is arranged, for example, in a flange, in particular the flange plate 29C or the coupling plate 29D.

FIG. 5A is a cross-sectional view with the section plane I of FIG. 4 and the viewing direction according to arrow I of FIG. 4. The section plane thus extends along the two rotation axes R. The fixed displacement pump shown in FIGS. 5A and 5B differs slightly from the fixed displacement pump shown in FIG. 4. The “interior” is essentially identical. A relevant difference consists in the fact that in this embodiment the pump cover may be integrated. In the fixed displacement pump according to FIG. 5A, the pump housing 29 is again composed of multiple individual components, specifically 29A to 29E. In contrast to the variant according to FIG. 4, however, 29E in FIG. 5A designates a separate pump housing cover, which is not provided as a separate part on the face side visible in FIG. 4 but is integrated in the housing sleeve 29B. The leakage oil outlet is thus located in the housing sleeve 29B in FIG. 4, and in the pump cover 29E in FIG. 5A. The pump cover can be secured with the aid of the connecting screws 32. As can be seen in FIG. 5A, in the present embodiment example the fixed displacement pump 9 likewise comprises two individual fixed displacement pump units 9A and 9B. However, the present invention obviously also comprises fixed displacement pumps having only one fixed displacement pump unit, for example comprising the elements 29A, 29C and 29E with a gearwheel pair. It is, however, also possible that only one fixed displacement pump unit or more than two fixed displacement pump units are provided.

In the present embodiment example, each fixed displacement pump unit comprises an intermeshing gearwheel pair 31A, 31B as a conveying element 30. The gearwheels are external gearwheels. The fixed displacement pump is driven via the input shaft 17 rotating about the rotation axis R (R1), which may be integral with the gearwheel 31A of the first fixed displacement pump unit 9A. What is essential is that the input shaft 17 and the first gearwheel 31A are connected to each other for co-rotation. The meshing engagement of the gearwheel 31A with the teeth of the second gearwheel 31B causes the second gearwheel 31B to rotate about the rotation axis R2. Each gearwheel 31A and 31B comprises a bearing axle 33 on each side in axial direction of the respective rotation axis R1 and R2, which protrudes in axial direction beyond the respective gearwheel 31A/31B, said bearing axles being marked in FIG. 5A with the index numbers 33.1 to 33.8 for better allocation. The bearing axle 33.1 protrudes via the input shaft, which protrudes beyond the pump housing 29. Alternatively, it is also possible to enable a connection to a drive device not via a projecting shaft but, for example, via a bushing in which an input shaft engages. The bearing axle 33.2 is located opposite the bearing axle 33.1 of gearwheel 31A on the rotation axis R1. The gearwheel 31A engages the gearwheel 31B having the two bearing axles 33.3 and 33.4 projecting opposite each other in axial direction of the rotation axis R2. In a corresponding arrangement, the gearwheel 31C comprises the bearing axles 33.5 and 33.6, and the gearwheel 31D comprises the bearing axles 33.7 and 33.8. It should be pointed out that the bearing axle 33.2 of the first fixed displacement pump unit 9A is in drive connection via a connection pin 34 positioned in the bearing axle 33.2 for co-rotation about the rotation axis R1, which at the same time engages the bearing axle 33.5 of the second fixed displacement pump unit 9B for co-rotation. This results in a serial arrangement of the two fixed displacement pump units 9A and 9B with respect to the input shaft 17. The second fixed displacement pump unit 9B is thus driven via the first fixed displacement pump unit 9A and the co-rotational connection achieved via the connection pin 34.

The bearing axles 33 (specifically 33.1 to 33.8) are each supported for rotation about the respective rotation axis R1 and R2 in axle bearings 35 (in allocation to the bearing axles 33.1 to 33.8 in the axle bearings 35.1 to 35.8), which axle bearings 35 may, in particular, be configured as slide bearings. The gearwheels 31A to 31D may be axially secured with the aid of bearing rests 36A and 36B for the first fixed displacement pump unit 9A and 36C and 36D for the second fixed displacement pump unit 9B, which are arranged inside the pump housing 29. The bearing rests may form the axle bearings 35.1 to 35.8. The gearwheels 31A to 31D project in radial direction to the respective rotation axis R1 and R2 with respect to the radial extension of the respective axle bearings 35.1 to 35.8, so that they are positively prevented from any appreciable axial displacement along the respective rotation axis R1, R2 by the bearing rests 36.

During operation of the fixed displacement pump 9, hydraulic fluid leaks from the conveying chamber 38 in axial direction of the rotation axis R1 and R2 between the bearing axles 33 and the axle bearings 35. This leakage oil is collected in the collection chamber 22 inside the fixed displacement pump 9. For a more detailed illustration of the total extension of the collection chamber 22, FIG. 5B shows the chamber free of any contour lines extending into the viewing plane of the cross-sectional view. For further clarification, the connection pin 34 is omitted. In FIG. 5B, in the respective axle bearings 35.1 to 35.8, some of the exit points 39.1 to 39.8 of the leakage oil from the axle bearings 35.1 to 35.8 into the collection chamber 22 are shown, the latter being contiguous as a whole and configured as a cavity.

Via the exit point 39.1, leakage oil escapes, for example, via the slide bearing formed between the bearing axle 33.1 and the axle bearing 35.1. Analogously, this applies to the exit points 39.2 (between 33.2 and 35.2 [FIG. 5A]), 39.3 (between 33.5 and 35.3), 39.4 (between 33.4 and 35.4 [FIG. 5A]), etc. At the exit point 39.1, there is first a collection cavity section 22.1 surrounding the bearing axle 33.1 in a ring shape. This section is formed in the flange plate 29C and opens into a channel bore 22.2 which may likewise be formed inside the flange plate 29C. Via the channel bore 22.2, which is likewise part of the collection chamber 22, a connection is formed to a cavity 22.3 provided on a face side of the bearing axle 33.3 and essentially configured, for example, as a hollow cone, which opens into a passage bore 22.4 penetrating the bearing axle 33.3, the gearwheel 31B and the bearing axle 33.4 along the rotation axis R2. The leakage oil flows entering via 39.1 and 39.3 and combined in the collection chamber 22 flow along the direction of arrow P1 through the collection chamber section formed with the entirety of the bearing axles 33.3 and 33.4 and 31B and the boundary walls of the collection chamber section 22.4 rotating about the rotation axis R2. In the region of the coupling plate 29D, the collection chamber 22 opens into a subregion 22.5 which is stationary relative to the pump housing 29 and in which the exit points 39.4 and 39.7 are located. This is followed by a section of the collection chamber 22 similar to the already described collection chamber region 22.4, which is designated by 22.6 and likewise passes through the region of the bearing axles 33.7 and 33.8 rotating around the rotation axis R2 as well as the gearwheel 31D, which is open at both ends. Towards one face side, it opens into the further collection chamber section 22.7, which adjoins the end of the bearing axle 33.8 on the face side in axial direction of the rotation axis R2 and is formed by the pump cover 29E. Via the channel 22.8, a connection is formed between the collection chamber section 22.1-22.7 described above and the leakage oil outlet 23. Similar collection chamber sections 22.9 to 22.13 are further found at the exit points 39.2, 39.5 and 39.6, which can be collected in this subregion of the collection chamber 22 and can flow to the leakage oil outlet 23 in the direction of arrows P4, P5.

All in all, a contiguous collection chamber 22 is thus available inside the fixed displacement pump 9, via which the leakage oil flows escaping via the axle bearings 35 can be combined and discharged from the fixed displacement pump 9 separately from the high-pressure side and the low-pressure side of the pump. It is important that this arrangement does not necessarily require the use of a tandem arrangement as shown by way of example in FIG. 5B, but can obviously also be used for a fixed displacement pump 9 with only one fixed displacement pump unit or with more than two fixed displacement pump units. Moreover, it is noted as a precaution that the connection pin 34 in FIG. 5B, which is only indicated by a dashed line, connects the bearing axles 33.2 and 33.5 with each other in a co-rotational but non-sealing manner. This region, i.e., the free space between the connection pin and the bearing axles, is thus likewise part of the collection chamber 22.

To enable an efficient collection of the leakage oil as described in FIGS. 5A and 5B, it is important that the collection chamber is sealed against the high-pressure side and the low-pressure side. For this, a sealing device 40 is provided, which in the present embodiment example comprises multiple sealing units arranged in axial direction of the rotation axes R1 and R2 one behind the other between a) the flange plate 29C and the bearing rest 36A and the pump housing 29A, b) the coupling plate 29D, the bearing rest 36B and the pump housing 29A, c) the coupling plate 29D, the bearing rest 36C and the pump housing 29B and d) the pump housing 29B, the bearing rest 36D and the pump housing cover 29E. In the present embodiment example, the sealing device 40 includes a conventional housing seal 40A and a collection chamber seal 40B according to the exemplary embodiment of the present invention. For a more detailed illustration of the structure and the mode of action of the sealing device according to the present invention, reference is made in this connection to FIGS. 6 and 7, as described below.

FIG. 6 first shows the structure of a sealing device 40 according to the prior art, i.e., for a fixed displacement pump 9* which is not suitable for use in a closed hydraulic circuit but can only be operated in an open hydraulic circuit system. Structurally and/or functionally like components are designated with “*” with reference to the preceding statements on the design of the fixed displacement pump according to one embodiment the present invention. In addition to the housing seal 40A, a partial collection chamber seal 40B* is provided which seals the collection chamber 22 only against the high-pressure side of the fixed displacement pump 9*. In this fixed displacement pump 9*, leakage oil flows are discharged from the collection chamber 22* (FIG. 6 shows, for example, compartment 22.3*, which is located on the face side of the bearing axle 33.3* in axial direction R2) via grooves 41 to the suction side of the fixed displacement pump 9* on the side of pump inlet 11* (indicated in FIG. 6 by the two dashed arrows). The negative pressure required for this on the suction side of the fixed displacement pump 9* is generated by the conveying function of the fixed displacement pump 9* in an open hydraulic circuit, i.e., there may be only a very low pressure level here, in particular at a maximum of 3 bar. A sealing against the suction side is therefore not provided for functional reasons. This arrangement is not suitable for operation of the fixed displacement pump 9* in a closed circuit, since then the suction effect required on the pump inlet side can no longer be generated, i.e., the pressure on the low-pressure side is significantly higher, in particular higher than 10 bar.

Therefore, in contrast to the conventional pump shown in FIG. 6, the fixed displacement pump according to one embodiment of the present invention has a collection chamber seal 40B which is designed such that it seals the collection chamber simultaneously against the low-pressure side (i.e., towards the pump inlet side) and against the high-pressure side (i.e., towards the pump outlet side). In this embodiment example, the collection chamber seal 40B comprises two completely and closed circumferential recesses 42A and 42B, which are connected to each other via a connecting web 42C. The collection chamber seal 40B may be formed in one piece, for example as a sealing gland. It is, however, also possible to use multiple individual subunits. Each of the circular recesses surrounds a respective one of the two rotation axes R1, R2. The collection chamber seal 40B is clamped between the flange plate 29C and the bearing rest 36A. Further collection chamber seals 40B may be clamped between the bearing rest 36B and the coupling plate 29D, the coupling plate 29D and the bearing rest 36C, and the bearing rest 36D and the pump housing cover 29E (FIG. 5b ) The required clamping force may be generated via the connecting screws 32. Each collection chamber seal 40B thus has in common that it may be designed as a flat gasket and/or molded seal and/or seals the collection chamber 22 of one of the fixed displacement pump units 9A and 9B on the inside against the conveying chamber 38, either towards the high-pressure side and/or the low-pressure side of the fixed displacement pump 9. Further, leakage oil accumulating in the collection chamber is not routed to a suction side via a groove 41 or a similar design, as is usual in prior art fixed displacement pumps. Instead, the fixed displacement pump 9 according to one embodiment of the present invention has for this purpose a separate connection between the collection chamber 22 and the outside environment of the fixed displacement pump, in particular the leakage oil outlet 23. The collection chamber 22 may therefore also be pressureless (i.e., without pressure difference) with respect to the outside environment.

FIG. 8 now illustrates a possible arrangement of a fixed displacement pump 9 according to one embodiment of the present invention in an embodiment of a hydraulic circuit diagram. The fixed displacement pump 9 is in fluid communication with the valve device 19, comprising a basic valve block 20 with two functional module blocks 21 arranged thereon, via the supply line 15 and the return line 16. It is noted as a precaution that the fixed displacement pump can be operated in both conveying directions with respect to the closed hydraulic circuit 8, so that the supply line 15 then becomes the return line 16 and vice versa. The valve device 19 is followed in the conveying direction by the hydraulic consumer 12, which is in the present case a hydraulic motor. The latter is likewise in fluid communication with the valve device 19 via a supply line 15 and a return line 16. The basic valve block 20 comprises a pump inlet PA, a motor outlet MA, a motor inlet MB and a pump outlet PB. Moreover, a feed pump 43 is connected to the basic valve block 20 of the valve device 19 via a feed line 44. The task of the feed pump 43 is to draw hydraulic fluid from the tank 25 and feed it into the closed hydraulic circuit 8 to compensate for leakage oil losses. For this purpose, a feed connection S is provided on the basic valve block 20 of the valve device 19, which is connected to a feed line running inside the basic valve block 20. Further, a feed filter 45 is provided in the feed line. The closed hydraulic circuit is pressure-protected via the pressure-limiting valve 47. The feed line is connected inside the basic valve block 20 via a pressure-limiting valve 46 so it is pressure-protected against the supply line 15. Further, a further pressure-limiting valve 47 may be provided towards the return line 16. The valve device 19 may further comprise pressure-limiting devices, in particular in the form of a pressure-limiting valve 46′, for example in the basic valve block 20, which enable controlled discharge of excess hydraulic fluid from the closed hydraulic circuit 8, for example due to thermal expansion effects. One or more of the pressure-limiting valves 46, 46′ and/or 47 may be controlled.

Possible for the functionality of the hydraulic circuit shown in FIG. 8 are, in particular, the functional module blocks 21A and 21B attached to the basic valve block 20, through which the hydraulic fluid is successively passed from the fixed displacement pump 9 before being forwarded to the hydraulic consumer 12. FIGS. 9A to 9D show the valve device 19 with the basic valve block 20 and various attached functional module blocks 21. With the aid of the functional module blocks, such as described in more detail below, functionalities that are achieved with a variable displacement pump in conventional closed hydraulic circuits may be imitated even if the fixed displacement pump described above is used, for example switching the conveying direction at the hydraulic consumer, varying the conveying rate, etc.

The functional module block 21A according to FIG. 9A comprises, for example, an on/off function. By switching the functional module block 21A, hydraulic fluid fed into the valve device 19 via the inlet PA may be returned directly to the pump outlet PB, bypassing the connections MA and MB and thus bypassing a consumer connected to MA/MB, as in the switching position shown in FIG. 9A. If the valve of the functional module block 21A is switched to the other position shown in FIG. 9A, hydraulic fluid entering the valve device 19 through the inlet PA is routed to the outlets MA or MB, depending on the switching position of the functional valve block 21B, and is thus forwarded to the hydraulic consumer.

With the aid of the functional module block 21B, it is thus possible, for example, to achieve a reversal of direction towards a hydraulic consumer connected to the connections MA and MB, for example a hydraulic motor.

The line sections leading back to the outlet PB are further pressure-protected against a tank line with the outlet T via suitable pressure-limiting valves 46.

An alternative embodiment example according to FIG. 9B is the functional module block 21C in the form of a non-switchable pass-through bridge. The variant shown in FIG. 9B does thus not allow the direction to be switched for the subregion of the connected hydraulic circuit 8 downstream of the fixed displacement pump 9.

In contrast to the previous configurations, in FIG. 9C the functional module block 21A has been replaced by the functional module block 21D. The latter comprises a flow control valve, in particular an electrical proportional flow control valve. With the aid of the flow control valve, it is possible to divide the volume flow of the hydraulic fluid flowing into the valve device 19 into a portion supplied to the motor outlet MA and a portion supplied to the connection PB to the fixed displacement pump 9. In this manner, the function of a variable displacement pump with variable conveying volume can thus be imitated towards a hydraulic consumer. In this regard, in the embodiment example shown in FIG. 9C there is a fully stepless variability between the maximum conveying volume and a zero conveying volume with respect to both the conveying volume fed via the outlet MA and the conveying volume fed via the outlet MB. In addition, the functional module 21B may be used to select the conveying direction in the circuit section of the closed hydraulic circuit 8 between the connections MA and MB.

In the embodiment example according to FIG. 9D, the functional module 21D is finally combined with the functional module 21C. This embodiment, which is simplified compared to FIG. 9C, thus enables a variable conveying volume, however without the possibility of a reversal of the conveying direction in the circuit section between the connections MA and MB as described above.

FIG. 10 finally describes the configuration of a valve device 19, for example, if a tandem pump arrangement is used, as described, for example, in the previous figures. Each of the fixed displacement pump units has in this case an associated valve device 19. One of the two valve devices may have a plug element 48 which interrupts the connection to the tank line T via the pressure-limiting valve 46, as shown in the preceding embodiments. This valve may likewise be of modular design in order to make it as easy as possible to adapt the basic block 20 to the respective configuration.

FIG. 11 finally illustrates steps of a method according to an exemplary embodiment of the present invention. Initially, hydraulic fluid is conveyed 49 towards a high-pressure side of a closed hydraulic circuit, in particular a closed hydraulic circuit as described above, with the aid of a fixed displacement pump. The conveying 49 results in driving 50 of a hydraulic consumer, in particular a hydraulic motor, with the aid of the hydraulic fluid conveyed on the high-pressure side in the closed hydraulic circuit. The method further comprises returning 51 the hydraulic fluid from the hydraulic consumer, in particular the hydraulic motor, to the fixed displacement pump on a low-pressure side of the closed hydraulic circuit. It is now possible for the method according to the present invention that during steps 49 to 51, leakage oil is collected 52 inside the fixed displacement pump in a separate collection chamber sealed against the low-pressure side and also the high-pressure side of the closed hydraulic circuit, which is then drained off from the fixed displacement pump in step 53. The method further involves feeding 54 hydraulic fluid into the closed hydraulic circuit to compensate for the leakage oil branched off from the closed hydraulic circuit in steps 52 and 53. While the present invention has been illustrated by description of various embodiments and while those embodiments have been described in considerable detail, it is not the intention of Applicants to restrict or in any way limit the scope of the appended claims to such details. Additional advantages and modifications will readily appear to those skilled in the art. The present invention in its broader aspects is therefore not limited to the specific details and illustrative examples shown and described. Accordingly, departures may be made from such details without departing from the spirit or scope of Applicant's invention. 

What is claimed is:
 1. A fixed displacement pump for use for conveying a hydraulic fluid in a closed hydraulic circuit, comprising: a pump housing; at least one conveying element arranged inside the pump housing in a conveying chamber and rotatably driven about a rotation axis (R1, R2) for conveying purposes, the conveying element having at least one bearing axle extending in axial direction of the rotation axis (R1, R2), the conveying element being supported for rotation with the bearing axle about the rotation axis (R1, R2) within an axle bearing provided in axial direction next to the conveying element and configured stationary relative to the pump housing; a fluid pump inlet via which hydraulic fluid to be conveyed can be conveyed from outside the fixed displacement pump into the conveying chamber on a low-pressure side; a fluid pump outlet via which conveyed hydraulic fluid from the conveying chamber can be discharged from the fixed displacement pump on a high-pressure side; and a device for discharging a leakage portion of the hydraulic fluid escaping from the conveying chamber via the axle bearing, wherein the fixed displacement pump has a collection chamber adjacent to the axle bearing for the leakage portion of the hydraulic fluid escaping from the axle bearing, wherein a sealing device is provided which seals the collection chamber from the low-pressure side and the high-pressure side of the fixed displacement pump, and wherein the fixed displacement pump has a leakage oil outlet connected to the collection chamber for discharging the leakage oil from the fixed displacement pump.
 2. The fixed displacement pump according to claim 1, wherein the collection chamber is configured as a collection chamber that is pressureless with respect to the outside environment during operation of the fixed displacement pump.
 3. The fixed displacement pump according to claim 1, wherein the collection chamber has at least one of the following features: the collection chamber extends through components that are stationary relative to the pump housing and/or stationary relative to the at least one bearing axle; the collection chamber comprises an annular space surrounding the bearing axle, in particular completely; the collection chamber comprises a collection head section directly adjoining a face side of the bearing axle in axial direction of the bearing axle; the collection chamber comprises a collection passage opening passing completely through the bearing axle in axial direction from one face side to the other; the collection chamber comprises at least one connection channel that connects an annular space surrounding a bearing axle, or a collection head section, or a collection passage opening, with an annular space surrounding a bearing axle, or a collection head section, or a collection passage opening; and the collection chamber comprises a manifold into which multiple connection channels lead.
 4. The fixed displacement pump according to claim 1, wherein the collection chamber is formed partially in and/or on one of the following components of the fixed displacement pump: in the bearing axle, extending through the latter in axial direction; in the conveying element, extending through the latter in axial direction; in a bearing rest in which at least one bearing axle is supported; in a pump flange, as a surface recess on a side opposite the flanging side; in a pump cover, as a surface recess on the side opposite an outside of the pump cover; and in a coupling plate of a tandem pump.
 5. The fixed displacement pump according to claim 1, wherein the sealing device has at least one of the following features: the sealing device is formed in one piece; the sealing device comprises at least one sealing section completely surrounding the bearing axle radially; and the sealing device comprises two sealing sections, each of which completely surrounds a bearing axle radially, and a connecting section which connects the two sealing sections.
 6. The fixed displacement pump according to claim 1, wherein the fixed displacement pump is an external gearwheel pump, comprising: two adjacent, meshing gearwheels with a respective external toothing; two parallel bearing axles arranged at least partially at a same height in axial direction of the rotation axes (R1, R2), one bearing axle carrying one gearwheel and the other bearing axle carrying the other gearwheel; and that the fixed displacement pump has at least one of the following features: one of the bearing axles is designed as a drive axle protruding from the housing, comprising a positive locking device for engagement of a drive arrangement, in particular a spline gearing; and only one of the bearing axles, the bearing axle driven by the other bearing axle, comprises a passage opening extending in axial direction completely through the bearing axle and open at both face sides as part of the collection chamber.
 7. The fixed displacement pump according to claim 1, wherein the fixed displacement pump is configured as a tandem pump with a first and a second fixed displacement pump unit, the tandem pump having at least one of the following features: wherein a coupling plate is provided which is arranged directly between two gearwheel pairs of the first and second fixed displacement pump units, and wherein a part of the collection chamber is formed by the coupling plate and/or a respective seal is provided as part of the sealing device on both sides of the coupling plate in the direction of the rotation axis (R1, R2) of the gearwheel pairs; wherein multiple bearing axles have passage openings passing completely through them as part of the collection chamber; and the tandem pump comprises a single common leakage oil outlet, in a pump cover.
 8. The fixed displacement pump according to claim 1, wherein the fixed displacement pump comprises a valve device connected to the fluid pump inlet and the fluid pump outlet and/or arranged upstream of the fluid pump inlet and the fluid pump outlet, the valve device having at least one of the following valve functions: switching between a direct and closed bypass from the fluid pump inlet to the fluid pump outlet and an interruption of the closed bypass; switching a fluid connection between the fluid pump outlet and a first outlet of the valve device to a fluid connection between the fluid pump outlet and a second outlet of the valve device; pressure protection of a high-pressure side of the fixed displacement pump and release of a discharge line when a pressure threshold on the high-pressure side is exceeded; establishing a pressure-protected fluid connection from a feed line to a low-pressure area of the fixed displacement pump; and dividing the hydraulic fluid flow fed into the functional module block into two partial flows.
 9. The fixed displacement pump according to claim 1, wherein the valve device is configured as a modular unit with: a basic valve block (20) comprising at least: a connection opening for connection to the fluid pump inlet; a connection opening for connection to the fluid pump outlet; a connection opening for connection to a motor inlet; a connection opening for connection to a motor outlet; a connection opening for connection to a feed line; and a line interruption device with an interruption outlet opening and an interruption inlet opening; and with a functional module block having a module block inlet opening and a module block outlet opening which is in fluid communication with the module block inlet opening via a valve device and/or a line connection, the functional module block being attached, interchangeably, to the basic valve block such that the interruption outlet opening is in fluid connection with the module block inlet opening and the module block outlet opening is in fluid connection with the interruption inlet opening.
 10. The fixed displacement pump (9) according to claim 9, wherein the functional module block is configured for performing at least one of the following functionalities: switching between two inlet and two outlet openings; static pass-through; switching on/off a feeding function; and dividing the hydraulic fluid flow fed into the functional module block into two partial flows.
 11. The fixed displacement pump according to claim 9; wherein the basic valve block is configured for simultaneous connection of two functional module blocks, for performing at least one of the following functionalities on a closed consumer, a hydraulic motor: on/off function; variable drive volume; and/or reversal of direction.
 12. The fixed displacement pump according to claim 9, wherein the valve device, the basic valve block, has a connection for a feed pump, and that the basic valve block has a pressure-protected, switching device for discharging hydraulic fluid from the closed circuit into a discharge line leading to a line connection.
 13. A closed hydraulic circuit, comprising: a fixed displacement pump according to claim 1; a hydraulic consumer, a hydraulic motor, supplied with hydraulic fluid by the fixed displacement pump, with a fluid motor inlet and a fluid motor outlet; a hydraulic fluid supply line connecting the fluid pump outlet to the fluid motor inlet; and a hydraulic fluid return line connecting the fluid motor outlet to the fluid pump inlet.
 14. A construction machine, a road paver, with a closed hydraulic circuit according to claim
 13. 15. A method for operating a fixed displacement pump operated in a closed hydraulic circuit according to claim 13, comprising the steps of: a) conveying hydraulic fluid towards a high-pressure side of the closed hydraulic circuit with the aid of the fixed displacement pump; b) driving a hydraulic consumer, in particular a hydraulic motor, with the aid of the hydraulic fluid conveyed on the high-pressure side; c) returning the hydraulic fluid from the hydraulic consumer, the hydraulic motor, to the fixed displacement pump on a low-pressure side of the closed hydraulic circuit; d) collecting leakage oil inside the fixed displacement pump in a separate collection chamber sealed against the low-pressure side and the high-pressure side of the closed hydraulic circuit; e) draining off the leakage oil from the fixed displacement pump; and f) feeding hydraulic fluid into the closed hydraulic circuit to compensate for the leakage oil branched off from the closed hydraulic circuit in steps d) and e).
 16. The method according to claim 15, wherein the collecting of leakage oil is performed at least partially via a passage opening arranged in a bearing axle.
 17. The method according to claim 15, wherein for collecting leakage oil in the collection chamber, the collection chamber is sealed against both the high-pressure side and the low-pressure side of the fixed displacement pump with the aid of a sealing device.
 18. The method according to claim 15, wherein the fixed displacement pump is configured as a tandem pump with a first fixed displacement pump unit and a second fixed displacement pump unit, wherein step d) includes combining the leakage oil flows of the first and second fixed displacement pump units and step e) includes draining off the combined leakage oil flows from the tandem pump together. 